1. Field of the Invention
The present invention relates to a whip antenna of a telescopic type which is mainly used in a mobile radio unit, and more particularly to an antenna apparatus which is arranged to be capable of coping with a plurality of frequency bands.
2. Description of the Related Art
In recent years, there has been an increasing demand for mobile radio units such as a cellular telephone set. As antennas which are used for such mobile radio units, linear whip antennas which can be accommodated in main bodies of the portable units are widely used.
Hereafter, as a conventional example, a description will be given of the configuration disclosed in Unexamined Japanese Patent Publication (kokai) No. Hei. 1-204504 with reference to FIGS. 13 and 14. It should be noted that these drawings are shown as FIGS. 2 and 4 in Unexamined Japanese Patent Publication (kokai) No. Hei. 1-204504. In addition, the reference numerals in the drawings are identical to those used in the reference.
As shown in FIG. 13, when an antenna element 14 is pulled out from a main body 10 of a telephone set, a contact member 15 is in contact with a contact piece 21a. Accordingly, the antenna element 14 is connected to a matching circuit assembly 12. On the other hand, when the antenna element 14 is accommodated in the main body 10 of the telephone set as shown in FIG. 14, a contact member 16 is in contact with a contact piece 21b. Consequently, the antenna element 14 is connected to the matching circuit assembly 12. Thus, the antenna element 14 is connected to the matching circuit assembly 12 not only when the antenna element 14 is pulled out from the main body 10 of the telephone set, but also when it is accommodated in the main body 10 of the telephone set.
In the above-described configuration, if the impedance when the antenna element 14 is viewed from the matching circuit assembly 12 with the antenna element 14 pulled out from the main body 10 of the telephone set is assumed to be Z1, and the impedance when the antenna element 14 is viewed from the matching circuit assembly 12 with the antenna element 14 accommodated in the main body 10 of the telephone set is assumed to be Z2, and if the element length of the antenna element 14, the feeding-point position, and the dimensions of a casing of the radio unit, and the like are configured such that Z1 becomes equal to Z2, then it is possible to obtain a favorable matched state by virtue of the matching circuit assembly 12 even in cases where the antenna element 14 has been pulled out from the main body 10 of the telephone set and it is accommodated in the main body 10 of the telephone set. Consequently, high-quality and stable mobile communication is possible.
However, in conjunction with the diversification of mobile communications, frequency bands which are used have also become diversified including, for example, an 800 MHz band, a 1.5 GHz band, and a 1.9 GHz band. For this reason, there has been a demand for radio units capable of jointly using systems with different frequency bands. In contrast, conventional antennas are adapted to cope with only one frequency band. Hence, if such an antenna is used in a radio unit which is capable of jointly using a plurality of systems, its characteristics deteriorate appreciably.
FIG. 15 shows the frequency characteristics of impedance when the antenna element 14 is viewed from the matching circuit assembly 12 with the antenna element 14 pulled out from the main body 10 of the telephone set and with antenna element 14 accommodated in the main body 10 of the telephone set. The graph shown in FIG. 15 is called a Smith chart, wherein the ranges R=0 to +.infin. and X-.infin. to +.infin. under the impedance Z=R+jX are mapped in a unit circle, and this chart is popularly used to indicate the impedance. The solid line in the chart shows the locus of impedance Z1(f) when the antenna element 14 is viewed from the matching circuit assembly 12 with the antenna element 14 pulled out from the main body 10 of the telephone set. Meanwhile, the broken line shows the locus of impedance Z2(f) when the antenna element 14 is viewed from the matching circuit assembly 12 with the antenna element 14 accommodated in the main body 10 of the telephone set. In addition, the marker shown by a filled circle (.cndot.) shows the impedance of the center frequency fA of the frequency band A, while the marker shown by a cross (x) shows the impedance of the center frequency fB of the frequency band B.
As shown in FIG. 15, Z1(f) and Z2(f) depict different loci due to the differences in the feeding position of the antenna element 14 and the surrounding environment. For this reason, even if the element length of the antenna element 14 and the dimensions of the casing of the main body 10 of the telephone set are determined such that Z1(fA)=Z2(fA) at the center frequency fA in the frequency band A, the impedance at the center frequency fB in the frequency band B becomes such that Z1(fB).noteq.Z2(fB). For this reason, only one matching circuit can be prepared with respect to two antenna impedances in the state in which the antenna element 14 is pulled out from the main body 10 of the telephone set and in the state in which it is accommodated in the main body 10 of the telephone set. Hence, there have been problems in that a favorable matched state cannot be obtained in either one state or in both states, that the modulation accuracy and reception sensitivity deteriorates, and that the communication quality becomes aggravated.